I don't think it's much of a real issue. The ambiguity, such as it is, is that the original D could have been served much sooner than the expiration of the 4(m) window, so you could argue that if the party being added received notice after service of the original D, it would be outside the original 4(m) window. I think Wright & Miller say though that this is bogus, as long as the added party has notice within the theoretical 120 day window which starts from original filing, it should be fine.

Say statute of limitations is 500 days. You file a claim on day 100. That means that you have 120 days to serve D. But say you served the wrong guy. The right guy can only relate back to that 120 day window. This makes no sense because you are prohibited from suing the guy, even though you have 280 days left before the statute of limitations runs.